Nebulizers, or atomizers as they are sometimes called, are devices that generate a fine spray or aerosol from a liquid. A particularly useful application for nebulizers is to provide a fine spray containing a dissolved or a suspended particulate drug for administration to a patient by inhalation.
Piezo-mesh based nebulizers are commonly used to generate aerosols in such drug delivery apparatus, whereby a piezoelectric element vibrates the liquid or a mesh or nozzle plate to produce the fine aerosol spray. In the latter case, droplets dispensed on the nozzle plate are vibrated by the piezoelectric element to create the spray.
FIG. 1 shows an exemplary nebulizer 2. The nebulizer 2 comprises a body 4 having an inlet 6 and an outlet 8 arranged so that when a user of the nebulizer 2 inhales through the outlet 8, air is drawn into and through the nebulizer 2 via the inlet 6 and outlet 8 and into the user's body. The outlet 8 is typically provided in the form of a mouthpiece or a facial or nasal mask or in a form that is suitable for connection to a separate replaceable mouthpiece or facial or nasal mask.
The nebulizer 2 comprises a reservoir chamber 10 between the inlet 6 and outlet 8 for storing a liquid 12, for example a medication or drug, to be nebulized (i.e. to be turned into a fine mist or spray). The nebulizer 2 is configured such that fine droplets of the nebulized liquid 12 combine with the air drawn through the nebulizer 2 when the user inhales to deliver a dose of the medication or drug to the user.
An actuator 14 such as a piezoelectric element is provided for agitating or vibrating the liquid 12 stored in the reservoir chamber 10 along with a nozzle plate 16 for nebulizing the liquid 12 when the liquid 12 is vibrated.
The nozzle plate 16 is typically in the form of a mesh or membrane having a plurality of small holes or nozzles through which small amounts of the liquid can pass.
In order for a particular medicine to be therapeutically effective when inhaled, the aerosol droplet size of the medicine must be within a narrow therapeutic range. This narrow range requires droplet sizes that are generated across the surface of the nozzle plate 16 to be substantially uniform. The size of the droplets is determined by the size of the nozzles in the nozzle plate 16. Ideally, each nozzle in the nozzle plate 16 should have the same size. Therefore, there are very fine tolerances on the size of the nozzles. Typically, it is desirable for the nozzles to have a diameter of 2.5 μm with a tolerance of +/−0.25 μm. There can be of the order of 5000 nozzles in a typical nozzle plate 16.
FIG. 2 is diagram illustrating the fabrication of a nozzle plate 16 according to a conventional fabrication process. The nozzle plate 16 is fabricated by depositing or growing a material 18 (such as a metal) on a substrate 20 around a plurality of mandrels 22 (only one of which is shown in FIG. 2). Metal 18 is deposited on the substrate 20 until it ‘spills over’ the top of each mandrel 22 (the ‘spill over’ portions being labeled 18′ and 18″) and forms a nozzle 24 generally in the middle of the mandrel 22. The mandrel 22 and substrate 20 are removed leaving a nozzle plate 16.
It can be seen that the size (diameter d) of the nozzle 24 obtained by the fabrication process is dependent on the thickness t of the metal 18 over the top of the mandrel 22, and therefore small variations to the growth of the metal layer 18 from a desired amount can result in large variations in the size of the nozzle 24. In addition, there can be local variations in the growth of the metal layer 18 across a nozzle plate 16 and also across multiple nozzle plates 16 on a substrate 20.
For example, if a typical overgrowth thickness t of the metal layer 18 on the mandrel 22 is 30 μm and a target diameter for the nozzle 24 is 2.5 μm, a 2% error in local thickness will result in a nozzle diameter variation of twice 2% of 30 μm, which is 1.2 μm. This equates to a relative error in the size of the nozzle 24 of (1.2/2.5)=48%, which is not acceptable. In fact, in practice it is difficult to achieve just a 2% variation in local thickness t.
To mitigate these difficulties, conventional techniques exert precise control over the processing conditions and attempt to equalize these conditions for all nozzles being formed on a substrate. However, even with this precise control, the production yield of a nozzle plate fabrication process is only around 10%.
There is therefore a need for a method for improving the yield of a nozzle plate fabrication process and an apparatus for implementing the same.